This invention relates to the prevention of the pollution of navigable waters and adjoining loci which occurs when polluting fluids leak from tanks of sinking and sunken vessels.
Liquid cargo and bunker tanks aboard a ship as parts of the ship's structure have long been provided with vents and overflow pipes, mainly to enable the escape of air from the tanks during the filling and vacating operations, to prevent harmful vacuum conditions during off-loading, and to avoid the buildup of high pressures in the tanks during loading. The overflow pipes also keep the ship's structure from being subjected to excessive pressures in the event of overfilling. Usually, the overflow pipes terminate in a gooseneck about three or four feet above the deck, depending on the depth of the tank. Some vent pipes may terminate at higher levels to avoid the accumulation of noxious or explosive gases at the deck level.
These vents and overflow conduits provide free venting at all times, for heretofore no valves or other restrictions have been incorporated in them. The American Bureau of Shipping does, however, require a device for closing the openings in an emergency. This closure device has generally been located at the end of the gooseneck and typically has comprised a floating ball check or hinged flap, operable either by hand or by the force of a submerging wave washing over the deck. Wooden plugs have often been inserted during heavy weather. Other types of closures have also been in use, but all these closure devices have been meant solely to prevent the entrance of seawater during temporary conditions of heavy weather when green water washes over the decks. In addition, vents from tanks of gasoline and other volatile liquids have been provided with screen-type flame arresters.
The present invention, however, is also concerned with what happens when a ship sinks and carries down tanks containing liquid petroleum products, chemicals, or other liquids which may be a source of pollution. Should the ship sink, many of her tanks will remain intact structurally, and in fact, all may be intact if the sinking is solely due to flooding of reserve buoyancy compartments, such as the engine spaces or other dry compartments of the ship.
In the past, if a vessel were to sink while her vents were open, as more often than not was the case, then as she sank, water entered the tanks and replaced any air volume above the liquid, while that air escaped through the vents. Air that could not escape, being pocketed or trapped, was then compressed. As the ship sank to greater depths, water continued to enter the tanks, further compressing trapped air until the pressure inside the tank became equal to that of the surrounding sea. Water, being of higher density than the gasoline, oil, or other petroleum or chemical products, settled to the bottom of the tank, forcing the petroleum products up and making them flow out from the overflow pipe, as more water seeped in to replace the oil. Millions of gallons of petroleum product or other chemical could thus escape and pollute the water.
Such leakage obviously can become a serious source of oil pollution in sunken or partially submerged ships, with oil flowing from bunkers as well as cargo. Moreover, such cargoes and bunkers are lost.
The present invention is another approach to the solution of the same basic problem to which U.S. Pat. No. 3,868,921 of Seymour et al. was directed; it may also be considered as an improvement over the invention of that patent. Among the objects of the invention are those of providing an overflow antipollution valve which:
a. does not restrict the vent or overflow passage area or otherwise prevent the vent or overflow function under normal operating conditions,
b. acts as a closure device for vents of the type required by the American Bureau of Shipping,
c. also automatically closes the vent or overflow pipe when the vessel is submerged,
d. continues to admit water to enter the tank during the submerging process to equalize tank pressure with that of the surrounding sea, in order to avoid collapse of the ship's structure due to overpressure,
e. once equilibrium is reached--that is, the vessel is on the bottom with sea and tank pressures equalized--is not opened by transient pressures from waves at the surface,
f. operates independently of the ship's orientation,
g. provides means for attaching a hose to enable a salvage ship to pump liquids out from the tanks of a sunken vessel, and
h. can be installed on existing vessels with minimum alteration to the ship's structure.